This invention comprises an improvement in the two level system shown by U.S. Pat. No. 4,339,633 issued July 13, 1982 and U.S. Pat. No. 4,379,950 issued Apr. 12, 1983 both to F. Ahmed. In a system of the type shown by the patents called a digital hybrid PBX/key system, each station instrument of the multiple line type has four conductors accessing the system, two control conductors and two speech conductors. Two wire telephone instruments may also be used, the two wires being the speech or voice leads. The speech conductors have access to a common pulse code modulation system (PCM) bus for the transfer of speech and certain control data between the plural, multi-port groups of the system and the system controller. The system bus commonly serves a plurality of groups of ports, each port being either a station, trunk or attendants cabinet. Idle time slots on the PCM bus are assigned by the system controller for use by a port in completing a call, with two time slots on the bus being assigned for each call.
Once a pair of time slots has been assigned to a call, the speech data in digital form is transmitted over the PCM bus between time slot interchangers (TSI), one such interchanger for each port involved in the call.
At the system level there is provided a system controller comprised of a processor which may either be the 8086 or 8088 microprocessor with associated memory. Each group of the system is equipped with microprocessor preferably of the 8086 type along with its associated memory.
Call processing is handled by a port accessing its group control over its control conductors. Request is made for the allocation of a pair of time slots on the PCM bus for the implementation of the call from the port. Available time slots are assigned and calling information is generated at the port and transmitted using the allocated time slots. The system controller processes calling information over the voice conductors and completes the call to a called station which may be a station connected to another port of the system, or may be a station outside the system using a trunk port to exit the system, the call being completed over the two allocated time slots for speech transmission over the speech conductors in PCM format. Call supervision is generally accomplished over the voice conductors.